Recently, various composite semipermeable membranes wherein a semipermeable ultrathin film is formed on a porous substrate have been proposed as semipermeable membranes for reverse osmosis or ultrafiltration. For example, a composite semipermeable membrane wherein a dense ultrathin film composed of polyethyleneimine crosslinked by a polyfunctional crosslinking reagent such as isophthaloyl chloride is formed on a porous substrate has been disclosed in U.S. Pat. No. 4,039,440. However, this composite semipermeable membrane is insufficient in oxidation resistance, particularly, in chlorine resistance, and there are disadvantages, in the case of the solution to be processed being sterilized, that the processing system is generally complicated and the processing costs become high because of the requirement for a dechlorination step prior to the membrane processing step. On the other hand, a composite semipermeable membrane wherein an ultrathin film composed of ethylenediamine modified polyepichlorohydrin crosslinked with the same crosslinking reagent as described above is formed on a porous substrate to improve chlorine resistance has been disclosed in U.S. Pat. No. 4,005,012. However, this composite semipermeable membrane has a problem in practical use because the chlorine resistance is only slightly improved as compared to U.S. Pat. No. 4,039,440, and the water permeability is small. Likewise, it has been disclosed in U.S. Pat. No. 3,951,815 that a semipermeable ultrathin film can be formed by crosslinking polyvinyl alcohol with a crosslinking reagent such as isophthaloyl chloride. However, this composite semipermeable membrane has a fault in practical use that the chlorine resistance is also slightly improved and the water permeability is small.
Further, the above described semipermeable membranes are not suitable for use in carrying out desalting of aqueous solutions of salts having a low osmotic pressure, such as brackish water or industrial water, etc., because useful performance of such membranes is only exhibited in the case of operating under a high operation pressure of at least 40 kg/cm.sup.2. That is, many of the known composite semipermeable membranes do not have a practically suitable water permeability, viz., at least as high as 0.5 m.sup.3 /m.sup.2 .multidot.day under a low pressure operating condition of from 10 to 15 kg/cm.sup.2.
On the other hand, composite membranes prepared by coating a porous substrate with amino compound monomers, such as phenylenediamine, piperazine, etc., and thereafter crosslinking with an aldehyde or diacyl chloride, have been disclosed in Office of Saline Water Research and Development Progress Report, PB-253193 (1976) and PB-288387 (1978). However, these membranes are not satisfactory from the viewpoint of water permeability, even though certain kinds of piperazine-acid chloride composite membranes are comparatively excellent in chlorine resistance and show a comparatively excellent performance under low pressure operating conditions. Also, it is difficult to completely cover fine pores on the substrate with the crosslinked polymer layer, because the amino compound monomer is applied to the porous substrate and thereafter it is polymerized by crosslinking. Consequently, membrane defects are easily caused, and composite semipermeable membranes having good reproducibility and performance are difficult to prepare.